Typically, fingerprints have a number of features including ridges, valleys, and finer points. A finer point includes a bifurcation where a ridge is branched and an end point where the ridge ends. The fingerprint can be regarded as a unique biometric data since its characteristic distribution is different every person and two people cannot have the same fingerprints. Thus, the use of biometric data such as fingerprints in a security system may ensure efficient and accurate protection for the district that requires security, movable assets or the like.
A fingerprint recognition apparatus for acquiring an image of the fingerprint may be classified into a semiconductor type apparatus and optical type apparatus, and an optical fingerprint recognition apparatus is widely used rather than a semiconductor fingerprint recognition apparatus. The optical fingerprint recognition apparatus includes a total internal reflection prism glass in which light proceeds through the incident surface of the prism, is reflected at the contact surface of the prism, and is emitted from the exit surface of the prism. When a finger is put on the contact surface of the prism, light undergoes diffused reflectance at the points where the ridges of the fingerprint are in contact with the glass. However, no light undergoes the diffused reflectance at the points where the valleys of the fingerprint are in contact with the glass. Thus, the light emitted from the exit surface of the prism exhibits the characteristics of the fingerprint.
The optical fingerprint recognition apparatus as described above has a merit of an excellent durability, but it has a demerit that a light source, a prism, a lens, and a sensor are arranged in certain distances and angles apart from each other, which renders it thick as compared with the semiconductor type apparatus and makes it hard to reduce the thickness.
Meanwhile, the research for the prisms used for optical fingerprint recognition apparatus has been continued so as to correct the distortion of a fingerprint image as well as to form and emit the fingerprint image. In the prism of the optical fingerprint recognition apparatus, because the distance between the image sensor and a fingerprint, i.e., an optical path is changed depending on the position of the fingerprint, the fingerprint image is distortedly imaged relative to a real image at the point where the fingerprint image is finally formed. In particular, trapezoidal distortions frequently occur.
In order for reducing the distortion of the image, a conventional optical fingerprint recognition apparatus combines a condensing lens for condensing the fingerprint image emitted from a prism with a separate correcting lens or a correcting prism, thereby correcting the distortion and aberration of the fingerprint image.
In this case, however, the conventional optical fingerprint recognition apparatus has the problem that an optical system increases in its volume. That is to say, such a conventional optical fingerprint recognition apparatus is unable to reduce its volume because a condensing lens, an image correction prism, and an optical path changing mirror are discretely installed, which additionally incurs a problem of a decreased productivity and an increased manufacturing cost.